KDevelop say hello to your new way of handling context menus.

This is a first simple implementation, without the weights or any other extra logic. I added a build entry to the project manager view context menu to demonstrate it. The docs on the Context class got updated with some hints what a plugin has to do.

Unfortunately for some unknown reason the signal mapper doesn't work, so only the dummy buildSelectedItem slot is called at the moment. If anybody can give me a hint whats wrong there that would be great